1. Field of the Invention
The present invention concerns a helium leak detector comprising a gas analyzer and a secondary pump comprising at least on its discharge side a Holweck type stage, said pump being of a type enabling the suction pressure to be reduced to a value at least equal to 10.sup.-4 mbar and its discharge pressure being sufficiently high to be compatible with the suction limit pressure of a dry primary pump comprising at least two stages.
2. Description of the Prior Art
A detector of this type is particularly advantageous because it does not comprise any oil seal pump and may be used in applications where the vacuum must be very clean. Also, in "sniffer" mode operation where the pumped flow is low, low-flowrate dry pumps are sufficient and have the advantage of being lighter and less bulky than oil seal vane pumps. Also, these pumps can operate easily in any position.
U.S. Pat. No. 4,893,497 describes a helium leak detector of this type.
In this document, if the monitored container leaks helium enters the detector at the discharge side of a secondary pump where the pressure is approximately 40 mbar for small leaks. This does not provide excellent sensitivity for very small leaks. Also, there is a pipe fitted with a purge valve at the discharge of the secondary pump, and therefore also at 40 mbar, for depolluting the detector for inspection of a subsequent container in the event that a leak is detected during inspection of a previous container.
Effected at this pressure level, the purge does not depollute the detector quickly at the level of the secondary pump.
An object of the present invention is to propose a detector of the type previously defined applicable to sniffer mode operation providing very good sensitivity and most importantly enabling very good and fast recovery of the detector in the event of pollution thereof resulting from a monitored container leaking.